Apart from the obvious distinction of a ring gag as the only kind of gag that leaves the wearer's mouth somewhat accessible and even allows the submissive to use his/her tongue to some extent, it makes kinky sexsafer in several ways:

If a long, cylindrical object were to be inserted into the mouth through the ring, the sub could not bite it, voluntarily or otherwise;

The sub is left with the ability to "say" a safeword (if that word consists chiefly of vowels); and

Most importantly, there is far less risk of suffocation for the wearer than with any other gag.

Many Dominants are not so much interested in muffling any sound that the sub may make as in rendering the sub incapable of speech. Speech is an important part of what makes humans distinct from animals, and removing this distinction can have profoundpsychological effects. In a BDSM scene, it can help push a sub into subspace. If this is the intended purpose of the gag, then a ring gag should definitely be considered.